1. Field of the Invention
The present invention relates to a shadow mask for use in a shadow mask type color cathode ray tube and a shadow mask type color cathode ray tube using such a shadow mask.
2. Description of the Prior Art
As shown in FIG. 1A, a shadow mask type color cathode ray tube comprises a panel 4 coated with a fluorescent film 3 in which three-color lengthwise successive fluorescent stripes (R, G, B) are arranged in many arrays in a direction perpendicular to scan lines of electron beams emitted from electron guns 1 through a shadow mask 2 to an inner surface of the panel, wherein a generally conical funnel 6 having a tubular neck portion 7 is connected to the panel 4 whereby a vacuum container is formed.
The three electron guns 1 are attached in an in-line arrangement in the neck portion 7. A deflecting yoke 5 for deflecting the electron beams is arranged on an outer periphery of the funnel 6. The shadow mask 2, in which slots 12 as shown in FIG. 1B for selectively transmitting the electron beams are formed, is arranged opposite to the fluorescent film 3 on the inner surface of the panel 4.
In the above-constituted shadow mask type color cathode ray tube, the three electron beams emitted from the electron guns 1 are deflected by a horizontal deflection magnetic field and a vertical deflection magnetic field generated by the deflecting yoke 5. The beams are then scanned over a fluorescent surface of the fluorescent film 3, and they collide with respective corresponding color fluorescent substances through the slots 12 in the shadow mask 2, whereby the fluorescent substances are excited and allowed to emit a light, so that a color image is displayed.
For such a shadow mask type color cathode ray tube, when the panel is covered with the striped fluorescent film 3 (hereinafter referred to as the "fluorescent stripe") parallel to a longitudinal direction of the slots 12 described above, generally used is an exposing method as disclosed in U.S. Pat. No. 4,049,451 in which a line source is arranged parallel to the longitudinal direction of the slots 12 and a perforated portion in the mask is used. However, in this method, due to a geometry of the shadow mask 2 and the panel 4 themselves, the light from the light source projected on the panel inner surface is rotated, and a light 13 passing through the shadow mask is inclined as shown in FIG. 2. Thus, a fluorescent stripe 9 constituting the resultant fluorescent film 3 is zigzag-patterned as shown in FIG. 2, so that an image quality is considerably deteriorated.
Various methods for improving the deterioration of quality due to this zigzag have been proposed. One of the methods is disclosed in U.S. Pat. No. 3,888,673 and 3,890,151 in which an exposure is performed by a combination of the rocking line source and a movable masking shield. However, in this method, although the exposure is performed by changing an angle of the light source and a position of the masking shield whereby an optimization of the exposure can be attempted over the inner surface of the panel, there is caused a problem in which an exposing time is considerably extended.
The further improved method is the method in which a negative meniscus lens 10 is arranged between a line source 8 and the shadow mask 2 as shown in FIG. 3, disclosed in Japanese Patent Application Laid-open No. 8-8060, No. 8-8061, No. 8-8062 or the like. This negative meniscus lens 10 has different curvatures on the inner and outer surfaces thereof. The negative meniscus lens 10 is intended for previously correcting an amount of rotation to a desired amount when the light passing through the slot 12 is emitted on the inner surface of the panel 4.
On the other hand, in the methods disclosed in Japanese Patent Application Laid-open No. 8-8060 through No. 8-8062 described above, when the fluorescent film 3 of the cathode ray tube is formed, the fluorescent film 3 must be formed so that it may match the position in which the electron beams emitted from the electron guns pass through the slots 12 in the shadow mask 2 and are emitted onto the inner surface of the panel 4. Thus, during the exposure, a correcting lens 11 having a complicated curved surface determined by a higher order function is arranged between the line source 8 and the shadow mask 2, whereby the optimization of the formation of the fluorescent film 3 is attempted. However, even if this correcting lens 11 is used, when the light passes through the correcting lens 11, the light, which passes through the negative meniscus lens 10 and is once optimized, is again changed. As a result, the light emitted onto the inner surface of the panel 4 is rotated, so that there is a shortcoming in which the zigzagged fluorescent stripe 9 is formed as shown in FIG. 2 and the zigzag is not sufficiently improved.